Campbell Figure Walkthrough: The lac Operon in E. coli -- Regulated Synthesis of Inducible Enzymes (HW 16 DNA gene regulation)

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Part B Binding of the lac repressor protein to the lac operator is regulated allosterically. This means that the presence of a specific small molecule that binds to the lac repressor controls whether the repressor can bind to the lac operator, and therefore whether the operon is turned on or off. Can you sort the following conditions based on whether they are associated with the lac operon being turned ON or turned OFF?

lac operon is ON: Allolactose is present. Allolactose binds to the lac repressor protein, causing it to change shape. The lac repressor protein cannot bind to the lac operator. RNA polymerase binds to the lac promoter and transcribes the operon genes. lac operon is OFF: Allolactose is absent. RNA polymerase cannot transcribe the operon genes. The lac repressor protein binds to the lac operator.

Part C E. coli live in your colon. The nutrients available to E. coli depend on what you eat. Since your diet varies, the nutrients available to E. coli also vary. Feedback regulation allows E. coli to activate operons only when the enzymes they make are needed. The lac operon described here is regulated by negative feedback. In negative feedback, a change in a system in one direction causes a response that counteracts the change. Negative feedback contributes to the maintenance of homeostasis, a stable internal environment.

The absence of lactose in the cell {represses} the operon, which permits the lactose level to {rise} if lactose becomes available in the environment. A rise in lactose level in the cell {activates} the operon, which causes the lactose level to {drop}.

Part D The video focuses on the lac operon, but there are others. For example, E. coli also have the trp operon, which has genes that code for enzymes necessary for synthesizing tryptophan, an amino acid necessary for protein synthesis. Like the lac operon, the trp operon has multiple genes under the control of one promoter. Operons can be either inducible or repressible. Inducible and repressible operons share some characteristics, but they are different in key ways. Can you identify whether the lac and trp operons are inducible or repressible? Sort each phrase to the appropriate bin.

Inducible operon Transcription can be induced in the presence of an inducer molecule lac operon Associated with catabolic pathways Transcription of the operon genes is usually OFF Repressible operon: Transcription can be repressed in the presence of a corepressor molecule trp operon Associated with anabolic pathways Transcription of the operon genes is usually ON Both operons: Negatively regulated by a repressor protein that binds to the operator

Part A Operons are structures in bacterial genomes that organize genes involved in a common metabolic pathway. A specific example is the lac operon, which produces three enzymes involved in the metabolism of lactose (milk sugar) as a food source for Escherichia coli. Identify the functions of each part of the operon and related structures. Drag the terms on the left to the appropriate blanks on the right to complete the sentences. Not all terms will be used.

1. The {lacZ} gene encodes the enzyme {β-galactosidase}, which hydrolyzes the disaccharide lactose into two monosaccharides: glucose and galactose. 2. The {lacY} gene encodes the enzyme {permease}, a membrane transport protein that facilitates the movement of lactose into the cell. 3. The {lacA} gene encodes {transacetylase}, an enzyme that detoxifies other molecules that might enter the cell through permease. 4. RNA polymerase binds to the {lac promoter} in order to transcribe the three lac genes. 5. The {lac operator} controls whether or not RNA polymerase can transcribe the genes. 6. The {lacI gene}, which lies just upstream from the lac operon, encodes the {lacI repressor} protein that binds to the operator to regulate the activity of the operon.